Variable frequency drives (VFD) are used to adjust motor speed of the pump by controlling the frequency of the electrical power supplied to the motor so as to regulate flow within a pump system. It is known in the prior art to use of a VFD and an external processor to control a centrifugal pump. The VFD is used to vary pump speed and provide speed and torque measurements. Typically, prior art VFD techniques require at least one external sensor (differential pressure, discharge pressure, or flow sensor) and use pump Affinity Laws to characterize (develop performance curve) normal pump performance at a number of different operating points. These expected normal values determined from the pump characterization process are stored in the processor's memory. Then, during pump operation, performance is again determined using the above method and compared by the processor to the corresponding stored “normal” values to determine if pump operation has become degraded.
In other prior art pump control methods, relationships (curves) are developed between TDH and Torque for minimum and maximum allowable flow points over a variety of speeds and used to identify the operating point of the pump and determine if it is operating within an allowable minimum and maximum flow range. Pump performance curves, relationships between BHP, flow and TDH, and between BHP, torque and speed, as well the Affinity laws are used to develop the TDH vs. torque curves. Motor torque and speed values from a VFD are supplied to a processor where TDH, torque and speed relationships are used in a processor to identify the operating point of the pump and determine if the pump is operating within the allowable minimum and maximum allowable flow ranges. This method has been deployed in a VFD.
Although the above prior art methods are adequate for their intended purposes, it would be useful to have a pump controller that neither requires an external sensor, nor does it require performance values calculated at multiple speeds to be stored in memory. Also, it would be useful to have a controller that does not require generation of any unique performance curves, i.e. TDH vs. Torque. Such features would simplify the set-up and operation of a VFD controller for a centrifugal pump.